As a form of a user interface, electronic devices comprising a display device having a touchscreen function, such as a mobile phone, a mobile information terminal and a personal computer have been developed. For such electronic devices having a touchscreen function, it is being considered to add a touchscreen function by attaching a separate touchscreen substrate to a display device such as a liquid crystal display device and an organic electroluminescent display device.
Currently, research is being conducted into a technique for producing an image-reading device by forming thin films from various materials on a transparent insulating substrate such as a glass substrate by means of chemical vapor deposition (CVD), etc., repeating cutting and grinding operations, etc., and forming a display element made from a scanning line and a signal line and an optical sensor element, etc.
In addition, a technique for detecting a contact position by means of a capacitive system is being investigated as a reading system of an image-reading device. In the capacitive system, a conductive electrode is allocated instead of an optical sensor element, etc. Based on the capacitance change between the electrode and a finger, etc., information of the finger, etc., on the surface of the panel is detected.
With respect to a display device using a capacitive system, people have vigorously developed a technique for incorporating a sensor function into a display panel such as a liquid crystal. This technique is a so-called in-cell technique. According to the in-cell technique, there is no need to attach a touchscreen which is separately manufactured to a liquid crystal, etc. Therefore, it is possible to avoid increase in thickness or weight of the whole electronic device. In addition, since there is no phase boundary between the liquid crystal, etc., and the touchscreen, light reflection which is easily caused at a phase boundary is not generated. Thus, the in-cell technique is excellent in display quality.
When a sensor function is incorporated into a display panel by using the in-cell technique, the display function is placed in proximity to the sensor function. Because of this structure, the detection signal of the sensor may be influenced by the display pattern of the display panel, and the detection accuracy might be decreased.
A plurality of thin-film transistors (TFTs) are used for the sensor circuit as an image-reading device which is incorporated into a display panel by means of the in-cell technique. In general, as the threshold voltages of TFTs differ from each other, it is difficult to produce several hundreds of thousands to several millions of TFTs having the same threshold voltage. As a result, the output signals from the sensor circuit vary from each other. This causes the reduction in detection accuracy.